Generally, in manufacturing multi-layered articles, such as integrated circuits (I.C.'s), silicon wafers are subjected to various processing steps. Due to these processing steps undesirable contaminants and residues may often be introduced onto the surface of the silicon wafers. These contaminants and residues may have a deleterious effect on subsequent processing steps and may lead to the ultimate production of I.C.'s which are unusable.
One point in manufacturing where contaminants may be introduced is the point where wafers begin processing. Many I.C. manufacturers grow ingots and slice wafers therefrom; others obtain pre-sliced wafers from outside sources. Regardless of the source of the wafers, it is likely that they contain some contaminants or residues of various types at this point.
In addition, as is well known, the manufacture of I.C.'s generally includes one or more photoshaping steps in which a photoresist is applied to, selectively exposed on, selectively removed from, and then used as a mask for etching of or deposition onto, the wafers. Usually at some subsequent time such photoresist masks must be removed by stripping solutions. These stripping solutions remove the photoresist to varying degrees. To the extent that some of the photoresist remains, contaminants or residues (relative to further processing) remain on the silicon wafers.
Additionally, after photoshaping, as well as other processing steps, cleaning and rinsing steps are often involved which are intended to remove contaminants and residues. Again, to the extent that such rinsing or cleaning steps are not properly effective, contaminants or residues remain on the surface of the wafers.
Thus, the undesirable contaminants or residues which may remain on the silicon wafer include photoresists and inorganic and organic contaminants and residues.
Because a single silicon wafer may have an extremely large number of circuits formed thereon, the presence of a contaminant or residue at one or more locations does not necessarily render all of the ultimately-formed circuits unusable or unworkable. Moreover in a large I.C. manufacturing facility, many wafers may be processed over a period of time and residues or contaminants present on one or more wafers do not necessarily adversely affect circuits formed on other wafers.
Accordingly, it is desirable to possess an analytical technique which permits one to evaluate the cleanliness of a single silicon wafer. To the extent that such technique indicates that contaminants or residues are present, one may conclude that good integrated circuits may not be formed at those locations. This does not, however, mean that good circuits cannot be formed at other locations on the wafer. In fact, a convenient analytical technique may well indicate that the majority of the surface of the silicon wafer will produce good circuits as well as pinpoint where good circuits cannot be formed.
Moreover, in the situation were numerous silicon wafers are being processed, an evaluation of single wafers on a random sampling basis may be used to evaluate statistically the probability that other wafers in a given lot will or will not permit the production of good integrated circuits.